Curious - Hager CU and SPD

[LeeJC]

I was under the impression it protects consumer side stuff from incoming supply surges. It would be difficult to believe it could protect consumer side devices fully when for example in the case of a ring final a surge at any accessory point is going to hit x number of other accessory points before the surge can be nulled at the CU.

And the reason for the 32A MCB was to protect the SPD from overcurrent, which in the case of Hager isn't now required seemingly.
Hence the original question

 

[ebee]

Answering EFLI.

Picture this,
you wire connections of the SPD to E and L outgoing and N outgoing of the DP mainswitch of the Consumer unit. That covers all outgoing MCB circuits from the consumer unit, in other words it quenches serious over-voltage right at the DP switch terminals (outgoing) .

On the identical installation next door you do exactly the same thing but with one slight difference , you put an MCB into the L of the SPD. If that MCB is in the ON position then effectively the two installations enjoy the same sort of protection from voltage spikes. If that MCB is in the OFF position then protection is lost. The MCB is protecting the SPD from overcurrent (if it is needed to do that)

 

[SUNRAY]

Well, very near adjacent circuits in the CU perhaps, as you get away from the CU depending upon the wavelength then you need another apparently say a DB feeding a DB feeding a DB situation you would need one in each DB is the way I understand it

OK so you are aware of the reasoning behind the position of the SPD, now think about the length of the leads between the supply cables via the MCB etc and the SPD.

 

[ebee]

OK so you are aware of the reasoning behind the position of the SPD, now think about the length of the leads between the supply cables via the MCB etc and the SPD.

A matter of millimetres?

 

[SUNRAY]

A matter of millimetres?

Yep millimetres.

And you mentioned wavelengths.

How many millimetres? let's say the leads supplied are about 250mm (the site doesn't specify so I'm making a guess from their picture) assuming they are installed without cutting down, and don't forget to include thelength of the MCB and the coil it contains adding a bit of inductance too.

So we have 500mm.

We would need to know the speed of the impulse and from that can calculate the delay in the wire, adding the MCB and inductance and from that we can calculate the additional impedance, then we have to add the reaction time of the SPD.

Now let's think about what a CU actually is, I'll make an analogy to a pond full of still water and throw a stone in creating ripples, think of the stone being the incoming power source and the accessories are dotted about the pond.
Something like this:

The red dot is the power source and the accesories are the green.
A surge on the supply creates the ripples and at some finite time a ripple hits the various accessories, including the purple dot, the SPD and at that point the surge is stopped, creating a void beyond it and that only exists in a circuit beyond the SPD (shown blue).
If the duration of the surge is long enough (ie; a longer wavelength and therefore a reduced impedance transformation) then yes it will alow a heavy current to flow in the resistance/impedance of the wiring for long enough to attempt to supress the surge further back along the circuit (pink circle) .

 

[ebee]

Yes Sunray, I`m not arguing this one with you. I have learned that the wavelength matters so making things more than just a simple _ we quenched it here so we do not now need to quench it downstream as well . I don`t know the answer and you may well have a valid point. I suppose that if we made an evaluation of what equivalent length the MCB makes we could always consider making the wire supplied with the kit that much shorter (if possible) But like I said I don`t know